Numerous types of mechanical and electrical pressure indicators are known in the prior art. This is also true of pressure indicators, generally known as restriction indicators, which are used to indicate flow restriction within an air filter. Restriction indicators are particularly useful for indicating a buildup of contaminants within air filter media, particularly within air filter media utilized in an internal combustion engine.
One type of restriction indicator which is used with internal combustion engine air filters is a maximum restriction indicator. In a maximum restriction indicator, an indicator device becomes locked in a single maximum position. The locking position of the maximum restriction indicator is calibrated to indicate when the air filter media has been clogged with an undesirable amount of contaminants. Thus, prior to such an indicator reaching its maximum locked position, the amount of contamination of the air filter media cannot be readily determined from the restriction indicator. U.S. Pat. No. 3,068,831 to Witchell; U.S. Pat. No. 3,258,960 to Baden et al; and U.S. Pat. No. 3,443,365 each disclose maximum restriction indicators.
Another type of restriction indicator is a continuous readout or dial gauge type. This type of restriction indicator can either provide a reading only when a vacuum is applied or can have a continuous locking mechanism for locking an indicator at a plurality of points. The continuous locking feature has the advantage of permitting an operator to determine the condition of an air filter media without turning on the internal combustion engine. The continuous locking readout indicators of which the applicant's are aware, however, do not provide for releasing of the locking mechanism from the same face of the indicator from which the indicator is read. Continuous readout type indicators are illustrated in U.S. Pat. Nos. 3,939,457 and 4,033,733 to Nelson and in West German Offenlegungsschrift 26 17 972.